1. Field of the Invention
The present invention relates to an electric switch having digital switch position detection including a switching device for contacting switch contacts associated with individual code tracks, different switch contacts being contacted by means of the switching device in different switch positions in totality and the tap off of the switch contacts contacted by the switching device being applied to an evaluation unit.
2. Background Art
Electric switches are known which can be brought into different switch positions. In the automotive industry, such switches can be associated with, e.g., a gearshift lever or also a cruise control device. These switches include as a switching device mostly a slider which extends over multiple code tracks. The code tracks bear individual switch contacts, it being provided that in different positions of the switching device and thus in different switch positions in totality, different switch contacts of the individual code tracks are contacted by the switching device.
The individual switch positions are distinguishable from one another in that a switch contact is contacted or not contacted depending on the position of the switching device. Such a switch contact is evaluated accordingly as a digital xe2x80x9c0xe2x80x9d or a digital xe2x80x9c1xe2x80x9d. Through appropriate combination of the contacting of the switch contacts in individual code tracks, the individual switch positions of the electric switch are distinguishable from one another in that each combination of contacted and non-contacted switch contacts is defined only once in the different switch positions.
The individual switch contacts are connected via in each case a tap line to an evaluation unit which is responsible for the read-out and evaluation of the switch contacts depending on the switch positions. Depending on the recognized switch position, a corresponding signal is then generated by the processor unit to trigger one or more actuators.
Even if an electrical switch of this sort basically functions in a satisfactory manner and can be manufactured with simple means, problems can arise in terms of unambiguous switch position detection if the contacting between a switch contact and the switching device takes place only in an insufficient manner, e.g., if the contact point is soiled. In such a case, it is possible that, due to the poor contacting, the contact resistance will be so large that the voltage level detected on the evaluation end lies below the specified threshold and incorrect switch position detection thus occurs as a result, even though it is not possible to detect that the read-out result is faulty. This is correspondingly true also in case of a line interruption.
Accordingly, there exists a desire to have an electric switch which enables reliable switch position detection, particularly also in terms of error detection.
Therefore, the object of the present invention is to further develop an electric switch as described in the introduction such that reliable switch position detection including error detection is possible.
This objective is solved according to the present invention in that depending on the position of the switching device in at least some switch positions the voltage level exhibits a different value to additionally distinguish this switch position.
In the electric switch according to the present invention, it is provided that the voltage level is not constant in different switch positions, but is used as an additional distinguishing criterion for the different switch positions. The benefit of this type of additional switch position encoding is that basically each switch position is first encoded digitally. Due to the additional and basically redundant information, it is possible to evaluate a switch position for the presence of an error because the voltage level of the contacted switch contacts in a switch position is evaluated in analog terms and it is different in each switch position. An electric switch is thus created in an easy manner for which the switch positions can be evaluated redundantly using digital and analog encoding. Error detection is made possible by the implemented analog evaluation.
In an electric switch of this sort, it can be provided that by means of the different voltage levels the digital xe2x80x9c0xe2x80x9d is divided into individual voltage levels, advantageously voltage level intervals corresponding to the number of switch positions to be implemented. Then, the digital xe2x80x9c1xe2x80x9d can be constant in terms of its voltage level. Analogously, an electric switch of this sort can also be structured inversely or using some combination.
To provide different voltage levels in the different switch positions, it is provided in a first embodiment of the present invention that, in the level track, in each case a differently encoded pull-down resistor is connected. In a second embodiment, it is provided to implement the level track line as a continuous potentiometer track which is connected with its one end to the operating voltage potential Vcc and with its other end to ground potential GND. Voltage level provision of this sort can be implemented in a particularly simple manner. The taps off of the individual switch contacts are applied to an A/D converter which is connected on the input side to a processor unit. Associated with the processor unit is a memory module in which the voltage levels or rather the voltage level intervals designating the different switch positions are stored.